The present invention relates to a device whose DC voltage section has two or more capacitors connected in series, and more particularly to a circuit configuration of a switching power supply circuit whose DC voltage section supplies a DC voltage of different voltage specifications.
A related prior art technique disclosed in, for example, FIG. 6 of Japanese Laid-Open Patent Publication No. 2000-60118 provides a switching power supply circuit which converts received AC power to a DC voltage and, from this converted DC voltage, produces a DC voltage of different voltage specifications. Specifically, this switching power supply circuit receives DC power from its DC voltage section and performs switching control on the primary side of the transformer by use of a PWM control circuit to output a DC voltage of different voltage specifications. The switching power supply circuit, however, is disadvantageous in that the voltage of its DC voltage section increases with increasing voltage of the received AC power (for example, such high receiving voltages as 200 V and 400 V). This means that the power loss produced in resistors 8 and 9 used in the starting circuit of the switching power supply circuit increases substantially in proportion to the square of the increase in the receiving voltage. To solve the above problems, the invention disclosed in Japanese Laid-Open Patent Publication No. 2000-60118 provides a method in which power is supplied to a power control IC used as the above PWM control circuit through a resistor only when the switching power supply circuit is started. The current flowing through the resistor is cut off after the switching power supply circuit has begun to operate. In this arrangement, however, the circuit for performing the cutoff control on the current must employ an active device which has a withstand voltage matching the voltage of the DC voltage section, resulting in a complicated circuit and increased cost.
Other methods have been also used to solve the above problems, as described in FIG. 6 of the above Japanese Laid-Open Patent Publication No. 2000-60118. Specifically, two or more resistors connected in series are installed in the starting circuit to divide the voltage; the dimensions of the resistor itself are increased to dissipate the heat produced in the resistor; the number of resistors connected in series is increased even when the voltage of each resistor does not exceed the maximum permissible voltage in order to spread the heat generation sources (resistors) in a wider area; or a sufficient heat dissipation space is provided around the resistor.
In the above switching power supply circuits, however, the voltage applied to the resistor of the starting circuit increases with increasing voltage Vi of the DC voltage section, causing the problems described in paragraphs (1) to (3) below.
(1) If the voltage or power to be applied to the resistor of the starting circuit exceeds its maximum permissible value, the resistor must be replaced by two or more resistors connected in series. The higher the voltage, the larger the number of the series-connected resistors, which is a factor in reducing the reliability and increasing the cost.
(2) Furthermore, the heat generated in the resistor due to the power loss increases with increasing voltage applied to the resistor, making it necessary to provide a sufficient heat dissipation space. This is a factor in increasing the equipment size and the cost.
(3) As for the related prior art technique of the invention disclosed in Japanese Laid-Open Patent Publication No. 2000-60118 which solves the above problems, it is disadvantageous in that the circuit for performing cutoff control on the current must employ an active device which has a withstand voltage matching the voltage of the DC voltage section. This means that an active device of high withstand voltage is required when the receiving voltage is high, which leads to an increase in the circuit complexity and the cost.
It is, therefore, an object of the present invention to provide a small and inexpensive switching power supply circuit with a simple circuit configuration which lowers the voltage applied to a resistor of its starting circuit to reduce the power loss, especially when the receiving voltage is high.